Emission-after-treatment devices, such as particulate filters, oxidation catalysts and NOx catalysts, require certain conditions in the engine exhaust in order to perform optimally. In particular, NOx catalysts have a relatively narrow temperature window within which the conversion efficiency is sufficiently high. Diesel particulate filters require sufficient oxygen in order to maintain regeneration at high engine loads. Prior to the present invention, the required exhaust gas conditions could not always be achieved during normal operation of the engine. In particular, at high engine loads, it was difficult if not impossible to provide sufficiently high levels of oxygen required by the particulate filter. Also the exhaust gas temperature could only be influenced to a certain degree by the combustion system.